1. Field of the Invention
This invention generally relates to the manufacture of a radiation-curable polyurethane such as a polyurethane acrylate oligomer by the reaction of a polyurethane prepolymer with an acrylic monomer. Radiation curable polyurethanes have application in, e.g., coatings, adhesives, sealants and photolithography.
2. Description of Related Art
Radiation-curable polyurethanes are well known and can be formed from isocyanate-terminated polyurethane prepolymer and an acrylic monomer with isocyanate-reactive groups such as hydroxyl groups. Acrylation of polyurethane prepolymers is widely used in UV-curable technology, see U.S. Pat. Nos. 4,775,727, 6,171,698 and 6,316,105, all of which are incorporated herein by reference in their entirety. Advantages of polyurethane acrylate oligomers include durability, excellent mechanical strength and superior abrasion resistance.
Radiation cure technology, i.e., cure by UV-light or electron beam, provides efficiency, environmental benefit (low VOC) and economy (low or acceptable cost of materials), as is evidenced by the growing application in adhesives and coatings technologies. Radiation curable formulations include acrylate oligomers, reactive acrylate diluents, photoinitiators and additives. Materials can cure in seconds, without polluting the air, and cutting costs. Other advantages include reduced energy consumption, greater productivity, single component materials, and room temperature cures (M. Szycher, “Szycher's Handbook of Polyurethanes,” CRC Press, 1999, p. 16-1).
While several ingredients are usually involved in the radiation cure formulations, acrylate oligomer is the major building block used to control the final cured properties. It is also usually the largest volume component, for example, 30-60% in coating applications.
Because of the versatile chemistry of polyurethane acrylates, it is possible to produce oligomers with a wide variety of properties. Modifications can come from the varieties of choices of isocyanates, polyol backbones, and acrylic monomers. Further modification of the backbone, such as varying the chain length, the level of unsaturation, and other functional parameters, will result in coatings with a variety of performance features. Other applications of polyurethane acrylates include for example, abrasion resistant formulations for PVC and floor tiles, wood coatings, overprint varnishes and printing inks. Due to their excellent adhesion and flexibility, they are suitable for a variety of flexible plastic substrates like plasticized PVC, polyester film, and polyurethane leather cloth. Polyurethane acrylates offer excellent toughness, chemical resistance, and adhesion to difficult substrates as well.
The isocyanate-terminated polyurethane prepolymer conventionally used is based on the reaction of a molar excess of diisocyanate monomer(s), e.g., aromatic diisocyanates such as diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI), or para-phenylene diisocyanate (PPDI) or aliphatic diisocyanates such as 1,6 hexamethylene diisocyanate (HDI), dicyclohexylmethane diisocyanate (H12MDI), isophorone diisocyanate (IPDI) or trans-1,4-cyclohexane diisocyanate (CHDI), with an organic polyol, e.g., polytetramethylene ether glycol (PTMEG), polyester, polycarbonate or polycaprolactone glycol, homopolymers and copolymers of ethylene oxide and propylene oxide (EO/PO).
While isocyanate-terminated polyurethane prepolymers can be reacted to form acrylate-endcapped oligomers for radiation curable applications, they are more commonly used without being reacted with an acrylate and without being radiation cured. More often, they are polymerized with a non-acrylate curative and without radiation, to form solid polyurethane elastomers. Such non-acrylate curatives are commonly aliphatic diols (e.g. 1,4-butanediol) or aromatic diamines (e.g. methylene-bis-ortho-chloroaniline).
The use of a molar excess of the diisocyanate monomers in forming the isocyanate-terminated polyurethane prepolymer leaves residual unreacted diisocyanate monomer, resulting in potential industrial hygiene issues. Accordingly, efforts have been made to convert diisocyanate monomer to polyurethane prepolymers with a reduced content of unreacted diisocyanate monomer. See, e.g., British Patent No. 1,101,410 and U.S. Pat. Nos. 4,182,825; 4,288,577; 4,892,920; 5,202,001 and 5,703,193. It is advantageous to have a polyurethane prepolymer with a reduced content of unreacted diisocyanate monomer in preparing polyurethane elastomers, in that, better hygiene, processing ease, and mechanical performance are achieved. Prepolymers from those diisocyanate monomers with the highest vapor pressures, hence the greatest hygiene concerns, e.g., TDI, PPDI, HDI, and IPDI, have been offered commercially in reduced unreacted monomer content from such sources as Crompton Corp., Baxenden, American Cyanamid Company and Air Products.
It is well known that both skin contact and inhalation of diisocyanate monomers must be avoided. As a result, a significant amount of attention has been given to the removal of unreacted TDI from prepolymers. Various methods to reduce the unreacted TDI content in prepolymers are known and disclosed in, for example, U.S. Pat. Nos. 3,248,372; 3,384,624 and 4,061,662. TDI prepolymers with less than 0.1% residual monomer are commercially available.
Such isocyanate-terminated prepolymers with a reduced content of free diisocyanate monomer have been known for at least 35 years. However, such prepolymers have not been previously endcapped with acrylates to form radiation-curable polyurethane acrylate oligomers. Only conventional prepolymers with an unreduced content of diisocyanate monomer have been used for this purpose. There has not been any known reason up to now to use prepolymers with a reduced monomer content, since acrylation converts any free diisocyanate monomer to its nonvolatile acrylate diadduct. The hygiene issue associated with the volatile free diisocyanate monomer in the starting prepolymer was thereby eliminated in the resulting acrylate-endcapped oligomer.
However, it has now been surprisingly found that there is an improvement in acrylate-endcapped oligomer when the starting isocyanate-terminated prepolymer is of the reduced free monomer type. The acrylate-endcapped oligomer has a significantly lower viscosity, and broader Newtonian viscosity plateau, i.e. viscosity remains essentially constant over a broader range of shear rate.
Radiation-curable compositions are advantageous because the materials are fast curing, low pollution and low cost. As stated above, radiation-curable polyurethane acrylate oligomers are one of the major components of these formulations, but polyurethane acrylates, like most radiation curable oligomers, are highly viscous. Diluents are thus generally required to make the thin film application possible for an end user.
There are various methods of reducing the viscosity of acrylate oligomers, see G. Webster, Chemistry & Technology of UV & EB Formulation of Coatings, Inks & Paints, Volume 2, p.259. One method is the addition of an organic solvent, which detracts from the many advantages radiation curable systems offer. Unfortunately, the solvents are a source of atmospheric pollution and can contribute to flammability. Another method is the addition of water to the formulation. There are certain advantages to this method, such as low cost, non-flammability and non-toxicity. However, there are several disadvantages such as poor compatibility with the oligomer and high heat of vaporization leading to difficulty in removing the water from the matrix. A further method is the addition of reactive diluents which are typically acrylic or methacrylic monomers. These reactive diluents are compatible with oligomers, will totally incorporate into the structure of the finished film and are widely used in current radiation curable industry. However, certain disadvantages remain such as the flammability and the toxicity of the diluents. These reactive diluents will participate in the reaction and alter the final properties of the finished film. While some of the effects on the final properties may be positive, others may not be desirable.
Various processes have been developed that attempt to reduce the presence of unreacted diisocyanate monomer content in polyurethane prepolymers. Among the various processes that have been developed in attempts to reduce the quantity of unreacted monomeric diisocyanate content in prepolymers are processes or methods that use falling film evaporators, wiped film evaporators, distillation techniques, solvent extraction, and molecular sieves. For example, U.S. Pat. No. 4,182,825 discloses a process to reduce the amount of diisocyanate (TDI) by distilling a prepolymer reaction product under vacuum conditions. U.S. Pat. No. 4,385,171 discloses a method for the removal of unreacted diisocyanate monomer (TDI) from prepolymers by codistilling the prepolymer reaction product with a compound that boils at a temperature greater than the boiling point of the diisocyanate. U.S. Pat. No. 5,703,193 discloses a process for reducing the amount of residual organic diisocyanate monomer (PPDI) in prepolymers by codistilling the reaction product in the presence of a combination of two inert solvents, with the first inert solvent having a boiling point below the boiling point of the diisocyanate monomer and the second inert solvent having a boiling point above the boiling point of the diisocyanate monomer. U.S. Pat. No. 4,061,662 discloses a process for the removal of unreacted toluene diisocyanate from prepolymers by passing the prepolymer reaction product through a column containing molecular sieves. U.S. Pat. No. 4,288,577 discloses the removal of unreacted methylene bis(4-phenyl isocyanate) (MDI) via solvent extraction with hexane.
Of these processes, distillation is a much simpler and more economical technique than solvent extraction or molecular sieve adsorption. There is no need to subsequently separate the monomer from either (flammable) hexane solvent or molecular sieves. However, in the distillation of diisocyanate monomers from polyurethane prepolymers, high temperatures must be avoided to prevent decomposition reactions in the prepolymer. Distillation without use of solvents is simpler still.
Of the polyurethane prepolymers with reduced unreacted diisocyanate monomer content that have been described, there remains a need for a radiation-curable composition such as a polyurethane-acrylate oligomer that is made from such a reduced unreacted diisocyanate prepolymer. Therefore, it would be advantageous to be able to produce a low-viscosity polyurethane acrylate oligomer without any of the above noted difficulties, while still maintaining the above stated advantages of polyurethane acrylate oligomers.